The present invention is directed generally to a valve device for a control cylinder, which is preferably of the type used for electronically controlled pneumatic actuation of the clutch of a motor vehicle.
In conventional single-acting pneumatic control cylinders in which the pneumatic piston is shifted into its initial position (zero position of the piston rod fixed to the piston) via a return spring, a specified actuation position is established by virtue of the pneumatic pressure prevailing in the piston chamber of the control cylinder. This means that the piston rod of the control cylinder is extended by a specified travel distance compared with its zero position. The air pressure in the control cylinder piston chamber determines the position of the control cylinder piston rod; the air pressure is lowered for retraction of the piston rod toward its zero position and raised for extension toward its maximum position allowed by the cylinder length.
The air pressure in the control cylinder piston chamber is varied by means of valves. In the simplest case, a switching pressurizing valve raises the air pressure and a switching venting valve lowers the air pressure.
For application of the control cylinder as an electronically controlled, pneumatically actuated clutch control cylinder as mentioned above, the pressurizing and venting valves are designed as electrically switched valves; the air pressure in the control cylinder piston chamber being varied as desired by the switching of these valves. For precision adjustment of a specified pressure or for establishing a specified pressure gradient, such as in the clutch-engagement process, the valves are actuated in a pulsed mode.
The control cylinder is connected to the motor vehicle clutch (which can be a push-type or pull-type clutch) in such a way that the motor vehicle clutch is completely disengaged in the piston rod zero position corresponding to a piston chamber pressure of zero. During an increase in the piston chamber pressure, the piston rod becomes extended, engagement begins at a specified piston rod position (clutch engagement point) and, beginning with a further specified position, the clutch is then completely engaged.
In the zero position of the control cylinder piston rod, in which, as explained, the piston chamber of the cylinder is depressurized and, also, the two valves are in unactuated position, it is possible that small leaks in the pressurizing valve in communication with the supply pressure can cause a gradual pressure buildup in the control cylinder piston chamber. This pressure buildup could be accompanied by undesired extension of the control cylinder piston rod from zero position, which could potentially lead to undesired engagement of the motor vehicle clutch, with the result that the vehicle might experience undesired movement under certain circumstances.
Undesired pressure buildup can be prevented by occasional actuation of the venting valve. For this purpose, however, the control electronics would require additional programming which may not be consistent with the program that controls the desired switching processes of the valve. Furthermore, additional functions may be required, for example, pressure sensing, that may not be needed for other purposes. Moreover, the system would then have to be continuously energized (current consumption, battery discharge). Such a solution is therefore not particularly advantageous.